1. Technical Field of the Invention
The present invention relates to a telecommunications network and, in particular, to the transportation of user data over A-Bis and A-interfaces within a public land mobile network.
2. Description of Related Art
With the continuing development of mobile telecommunications systems, such as the Global System for Mobile (GSM) communications standard, mobile subscribers and associated mobile terminals are able to communicate non-speech user data over the existing mobile telecommunications network at a much faster data rate than at a conventional speech data rate. Such data services include ISDN connections, facsimile transmission, modem connection, and other specified bearer services as set forth within the GSM specifications. As a result, a telecommunications module known as an Interworking Function (IWF) has been developed to enable the transmission and protocol adaptation of such user data from one telecommunications network to another associated telecommunications network. Accordingly, the IWF enables interconnection with networks such as Packet-Switched Public Data Networks (PSPDNs) or Circuit-Switched Public Data Networks (CSPDNs). Furthermore, it is also used when the connected network is simply the Public Switched Telephone Network (PSTN) or Integrated Service Digital Network (ISDN). Such an IWF may be implemented together with the mobile switching center (MSC) function, or it may be performed by separate equipment. Because the IWF communicates such data using a first protocol transmitted over a first data rate, and because a base transceiver station (BTS) serving a particular mobile station communicates the same data using a second protocol transmitted over a second data rate, another device known as a Transcoder/Rate Adaptor Unit (TRAU) is placed therebetween. A first communication link established between an IWF and a corresponding TRAU is known as an A-interface, and a second communication link established between the TRAU and a serving BTS is known as an A-Bis interface. The interconnecting TRAU then functions to facilitate the communication of data between the two incompatible communications links.
In accordance with the International Telegraph and Telephone Consultative Committee (CCITT) specifications, for example, data frames formatted as set forth in the CCITT V110 specification are communicated over the A-interface with a 9.6 Kbit/s user data rate. Similarly, data frames formatted as set forth in the GSM 08.60 specification are communicated over the A-Bis interface with a 13.5 Kbit/s user data rate. The interconnecting TRAU then performs the data transformation and adaption between the connected A-interface and the A-Bis interface.
With increasing demands of high capacity data communication within a mobile telecommunications network, mobile subscribers and associated applications are demanding a 14.4 Kbit/s user data transmission rate from the serving network. However, since the serving A-interface is capable of transporting only 9.6 Kbit/s user data, and the corresponding A-Bis interface is capable of transporting only 13.5 Kbit/s user data, a highly demanded 14.4 Kbit/s data connection with a mobile terminal is conventionally not achievable.
A number of solutions have been introduced to solve the above data payload problem. One such solution is to use "double TRAU" frames to communicate 14.4 Kbit/s data payload between the serving BTS and the IWF. Two consecutive TRAU frames formatted in accordance with the GSM 08.60 specification are used to accomplish 16 Kbit/s for the frames and to include 14.4 Kbit/s user data payload. Since each TRAU frame has a duration of 20 ms, the double TRAU frames are transmitted with a duration of 40 ms.
However, since all applicable modules, such as channel coding blocks, residing within the serving BTS are based on 20 ms data frames, by introducing 40 ms frames, the applicable modules need to be extensively modified and altered to buffer incoming data received from the TRAU and to delay transmitting outgoing data received from mobile terminals. Furthermore, since the GSM specification mandates communicating 20 ms frames over the A-bis interface, by introducing a different frame duration, general incompatibility problems are created within the serving mobile telecommunications network.
Accordingly, there is a need for a mechanism to transport 14.4 Kbit/s user data payload over the serving mobile telecommunications network while maintaining the 20 ms frame duration within the A-Bis interface.